Mirror World

The 'mirror world' was a warped, blurry, direction-reversed version of the human world that beings accessed via a mirror on Mallet Island. During the events of Devil May Cry, Dante encountered it twice: first, when his 'reflection' in the mirror, walked out of the mirror and into the human world, revealing itself to be Nelo Angelo, and second, when Dante himself crossed through the mirror to recover the Philosopher's Stone.

Trivia
The Mirror World was a fully reversed version of the human world. Its directions and structures were symmetrical to those in the real world, its sky showed beams of light returning into a crimson sun, and it apparently hosted Nelo Angelo, himself the corrupted twin of Dante.